Figure 1

(a) Illustration showing the etching process. (b) Atomic composition of the BK7 surface as mapped by XPS before and after the etching. (c) Cross-sectional and (d) top-view SEM images of etched BK7 glass. (e) Modeled refractive indices of the etched BK7 glass using ellipsometry data, the etched layer is equally divided into five slices in the model, showing a gradient index profile. (f) Calculated porosity of 5 slices based on effective medium approximation.